Page 38 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 2: Logic Gates

Figure 2.28: Tri-state Gate Control Notes

It is also possible to connect Tri-state Gate “back-to-back” to produce a Bi-directional Gate circuit
with one “active-high buffer” connected in parallel but in reverse with one “active-low buffer”.
Here, the “enable” control input acts more like a directional control signal causing the data to be
both read “from” and transmitted “to” the same data bus wire.
A digital gate is another single input device that does not invert or perform any type of logical
operation on its input signal as its output exactly matches that of its input signal. In other words,
its output equals its input. It is a “non-inverting” device and so will give us the Boolean expression
of: Q = A.
Then we can define the operation of a single input digital gate as being:
“If A is true then Q is true”
Table 2.11: Truth Table

Symbol Truth Table
A Q
0 0
A Tri-state Gate 1 1

Boolean Expression Q = A Read as A gives Q

The Digital Tri-state Gate can also be made by connecting together two NOT gates as shown below:
The first will “invert” the input signal A and the second will “re-invert” it back to its original level.
2.2.3 Double Inversion Using NOT Gates
Figure 2.29: Tri-state Gate Control

You may think “what is the point of a Digital Gate”, if it does not alter its input signal in any
way or make any logical operations like the AND or OR gates, then why not use a piece of
wire instead and that is a good point. But a non-inverting digital Gate has many uses in digital
electronic circuits, as they can be used to isolate other gates or circuits from each other or they
can be used to drive high current loads such as transistor switches because their output drive
capability is much higher than their input signal requirements, in other words gates are used for
power amplification giving them a high fan-out capability.

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